In PWM-controlling a motor through an inverter, for example, a transistor (switching element) at one of a high side (arm) and a low side (arm) is turned on and a transistor of a different phase at the other of the high side and the low side is turned on and off repetitively. When a transistor is turned off, a coil of the motor generates a counter-electromotive force so that a free-wheeling current flows in a free-wheeling diode connected to the transistor, which is in the same phase but at the other side of such a turned-off transistor.
Since the forward direction voltage of a diode is generally about 0.7 V, loss arises in correspondence to a free-wheeling current when a free-wheeling current flows in a free-wheeling diode. Patent document 1, for example, discloses a technology to reduce loss in a diode by turning on a drive transistor at a high side at the same time as turning off a drive transistor at a low side in PWM-controlling a DC motor by a H-bridge circuit, thereby flowing a free-wheeling current to a drive transistor without passing through a free-wheeling diode. Patent document 2 discloses a technology to perform the similar control for each switching element of an inverter circuit, in PWM-controlling a three-phase motor by an inverter circuit in a three-phase bridge configuration.    (Patent document 1) JP H9-18313A    (Patent document 2) JP 2005-9480A (US 2004/0234402A1)
Motor control by an inverter circuit is also used in a valve timing regulating apparatus, which regulates a relative phase between a crankshaft and a camshaft of an internal combustion engine by applying control torque generated by a rotary shaft of a motor to a phase regulating mechanism and driving the camshaft to rotate in accordance with the control torque. According to this control, the motor sometimes rotates in a direction, which is different from the direction instructed by a control circuit in a course of fall of the rotation speed of the internal combustion engine. That is, such a situation arises until the motor actually starts to changes its direction of rotation, when the motor is instructed to rotate in reverse while the motor is rotating in the forward direction.
In this situation, the motor operates as a generator for a moment. As a result, a free-wheeling current, which is greater than that flowing under normal PWM control, flows in a free-wheeling diode of a switching element forming an inverter circuit. FIG. 14 shows in (a), for example, each phase current of U-phase, V-phase and W-phase when a target rotation direction and an actual rotation direction of a motor agree (normal rotation) in case that a three-phase motor is energized according to 120° (120-degree) energization method. FIG. 14 also shows high-side phase currents and low-side phase currents in (b) and (c), respectively. Each hatched part in (c) indicates a current, which flows when the PWM control is performed by switching the low side. Each hatched part in (b) indicates a current, which flows as a free-wheeling current in correspondence to such a current. Each non-hatched part, which appears at the end of the hatched part as shown in (b), indicates a free-wheeling current, which flows in the high-side when the PWM control for low-side switching elements is switched to stop.
FIG. 15, on the contrary to FIG. 14, shows a situation, when the target rotation direction and the actual rotation direction of the motor differ (opposite rotation). In this case, since the direction of rotation of the motor becomes opposite to an energization pattern in an inverter circuit, a phase current is distorted as shown in (a). A current, which free-wheels in the high-side, becomes greater than a current, which flows in the low-side as a result of switching control. A hatched part in (b) indicates, similarly to FIG. 14, a free-wheeling current, which flows in the low-side switching elements in a switching period. Thus, loss in the free-wheeling diode increases. Since a current (indicated by a solid line), which flows in correspondence to a counter-electromotive force generated in a coil of a motor, is added to a free-wheeling current flowing at this time as shown in FIG. 16, the loss in the diode further increases. FIG. 16 schematically shows each switching element of an inverter circuit and coils of a motor. This figure specifically shows a situation, in which a free-wheeling current (indicated by a one-dot chain line) flows through a fee-wheeling diode connected to a V-phase high-side switching element, when a V-phase low-side switching element (indicated by X) changes its state from ON to OFF at the time of switching over the V-phase low-side and the W-phase low-side by a PWM signal while the U-phase high-side is turned on. A current flowing in the W-phase low-side is shown by a broken line. A cross-hatched part in (b) of FIG. 15 correspond to a period, in which a free-wheeling current caused by the above-described induced voltage is superposed. Thus loss arises in the free-wheeling diode.